Can We See the Energy Densities? II. Insights from Linear-Response Time-Dependent Density Functional Theory Calculations (original) (raw)
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The Journal of Chemical Physics, 2020
The time-dependent density functional theory (TDDFT) has been broadly used to investigate the excited-state properties of various molecular systems. However, the current TDDFT heavily relies on outcomes from the corresponding ground-state density functional theory (DFT) calculations which may be prone to errors due to the lack of proper treatment in the non-dynamical correlation effects. Recently, thermally-assisted-occupation density functional theory (TAO-DFT) [J.-D. Chai, J. Chem. Phys. 136, 154104 (2012)], a DFT with fractional orbital occupations, was proposed, explicitly incorporating the non-dynamical correlation effects in the ground-state calculations with low computational complexity. In this work, we develop time-dependent (TD) TAO-DFT, which is a time-dependent, linear-response theory for excited states within the framework of TAO-DFT. With tests on the excited states of H2, the first triplet excited state (1 3 Σ + u) was describe well, with nonimaginary excitation energies. TDTAO-DFT also yields zero singlet-triplet gap in the dissociation limit, for the ground singlet (1 1 Σ + g) and the first triplet state (1 3 Σ + u). In addition, the overall excited-state potential energy surfaces obtained from TDTAO-DFT also have excellent agreement with the results obtained from the state-of-the-art equation-of-motion coupled-cluster singles and doubles (EOM-CCSD) method especially for singlet excited states.
The Journal of chemical physics, 2004
Time-dependent density functional theory (TDDFT) calculations of charge-transfer excitation energies omegaCT are significantly in error when the adiabatic local density approximation (ALDA) is employed for the exchange-correlation kernel fxc. We relate the error to the physical meaning of the orbital energy of the Kohn-Sham lowest unoccupied molecular orbital (LUMO). The LUMO orbital energy in Kohn-Sham DFT--in contrast to the Hartree-Fock model--approximates an excited electron, which is correct for excitations in compact molecules. In CT transitions the energy of the LUMO of the acceptor molecule should instead describe an added electron, i.e., approximate the electron affinity. To obtain a contribution that compensates for the difference, a specific divergence of fxc is required in rigorous TDDFT, and a suitable asymptotically correct form of the kernel fxc(asymp) is proposed. The importance of the asymptotic correction of fxc is demonstrated with the calculation of omegaCT(R) fo...
Time-dependent density functional theory employing optimized effective potentials
The Journal of Chemical Physics, 2002
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Excitation energies of molecules within time-independent density functional theory
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Recently proposed exchange energy functional for excited-states is tested for obtaining excitation energies of diatomic molecules. The functional is the ground-state counterpart of the local-density approximation, the modified local spin density (MLSD). The MLSD functional is tested for the N2 and CO diatomic molecules. The excitation energy obtained with the MLSD functional for the N2 molecule is in close vicinity to that obtained from the exact exchange orbital functional, Krieger, Li and Iafrate (KLI). For the CO molecule, the departure in excitation energy is observed and is due to the overcorrection of self-interaction.
Triplet Excitation Energy Transfer with Constrained Density Functional Theory †
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We study the electronic coupling matrix element for triplet excitation energy transfer processes with a number of different computational methods. For the first time, constrained density functional theory (CDFT) is applied to the problem of energy transfer, and results are compared with direct coupling calculations of broken symmetry and fragment densities, as well as the splitting method. A naïve calculation of the electronic coupling using diabatic and adiabatic energy differences is shown to yield erroneous results due to the fractional spin error present in both Hartree-Fock and commonly used DFT exchange-correlation functionals. Some potential issues concerning the splitting method with triplet references within Hartree-Fock and DFT are discussed. We find that only methods that compute the matrix element directly (either from CDFT, broken symmetry, or fragment states) appear to be robust. Several illustrative examples are presented.
The Journal of Chemical Physics, 2013
Quantum-Mechanical methods that are both computationally fast and accurate are not yet available for electronic excitations having charge transfer character. In this work, we present a significant step forward towards this goal for those charge transfer excitations that take place between non-covalently bound molecules. In particular, we present a method that scales linearly with the number of noncovalently bound molecules in the system and is based on a twopronged approach: The molecular electronic structure of broken-symmetry charge-localized states is obtained with the Frozen Density Embedding formulation of subsystem Density-Functional Theory; subsequently, in a post-SCF calculation, the full-electron Hamiltonian and overlap matrix elements among the charge-localized states are evaluated with an algorithm which takes full advantage of the subsystem DFT density partitioning technique. The method is benchmarked against Coupled-Cluster calculations and achieves chemical accuracy for the systems considered for intermolecular separations ranging from hydrogen-bond distances to tens ofÅngstroms. Numerical examples are provided for molecular clusters comprised of up to 56 non-covalently bound molecules.
Chemical Physics Letters, 2004
The performance of time dependent density functional theory methods for the computation of electronic absorption spectra of molecular solutions is investigated using aqueous acetone as model system. Solute and solvent are treated at the same level of theory. Whereas transition energy and intensity for the intra-molecular 1 A 2 n fi p * transition are described to good accuracy by a conventional generalised gradient corrected exchange correlation functional (BLYP), explicit inclusion of exact exchange is found to be a necessary requirement to suppress overlap of the carbonyl band with spurious excitations involving transfer of electron charge from or to states with non negligible solvent character.